Thinking In C++. Volume 2: Practical Programming

  in.seekg(10);

  tellPointers(in);

  in.seekp(20);

  tellPointers(in);

  stringstream memStream("Here is a sentence.");

  memStream.seekg(10);

  tellPointers(memStream);

  memStream.seekp(5);

  tellPointers(memStream);

} ///:~

         37.             Suppose you are given line-oriented data in a file formatted as follows:

Australia

5E56,7667230284,Langler,Tyson,31.2147,0.00042117361

2B97,7586701,Oneill,Zeke,553.429,0.0074673053156065

4D75,7907252710,Nickerson,Kelly,761.612,0.010276276

9F2,6882945012,Hartenbach,Neil,47.9637,0.0006471644

Austria

480F,7187262472,Oneill,Dee,264.012,0.00356226040013

1B65,4754732628,Haney,Kim,7.33843,0.000099015948475

DA1,1954960784,Pascente,Lester,56.5452,0.0007629529

3F18,1839715659,Elsea,Chelsy,801.901,0.010819887645

Belgium

BDF,5993489554,Oneill,Meredith,283.404,0.0038239127

5AC6,6612945602,Parisienne,Biff,557.74,0.0075254727

6AD,6477082,Pennington,Lizanne,31.0807,0.0004193544

4D0E,7861652688,Sisca,Francis,704.751,0.00950906238

Bahamas

37D8,6837424208,Parisienne,Samson,396.104,0.0053445

5E98,6384069,Willis,Pam,90.4257,0.00122009564059246

1462,1288616408,Stover,Hazal,583.939,0.007878970561

5FF3,8028775718,Stromstedt,Bunk,39.8712,0.000537974

1095,3737212,Stover,Denny,3.05387,0.000041205248883

7428,2019381883,Parisienne,Shane,363.272,0.00490155

The heading of each section is a region, and every line under that heading is a seller in that region. Each comma-separated field represents the data about each seller. The first field in a line is the SELLER_ID which unfortunately was written out in hexadecimal format. The second is the PHONE_NUMBER (notice that some are missing area codes). LAST_NAME and FIRST_NAME then follow. TOTAL_SALES is the second to the last column. The last column is the decimal amount of the total sales that the seller represents for the company. You are to format the data on the terminal window so that an executive can easily interpret the trends. Sample output is given below.            Australia

              ---------------------------------

*Last Name*   *First Name*   *ID*    *Phone*        *Sales*   *Percent*

Langler       Tyson          24150   766-723-0284     31.24   4.21E-02

Oneill        Zeke           11159   XXX-758-6701    553.43   7.47E-01

(etc.)

5: Templates in depth

The C++ template facility goes far beyond simple "containers of T." Although the original motivation was to enable type-safe, generic containers, in modern C++, templates are also used to generate custom code and to optimize program execution through compile-time programming constructs^.

In this chapter we offer a practical look at the power (and pitfalls) of programming with templates in modern C++. For a more complete analysis of template-related language issues and "gotchas," we recommend the superb book by David Vandevoorde and Nico Josuttis.[49] 

Template parameters

As we illustrated in Volume 1, templates come in two flavors: function templates and class templates. Both are wholly characterized by their parameters. Each template parameter itself can represent one of the following categories of arguments:

1.       Types (either built-in or user-defined)

2.      Compile-time constant values (for example, integers, and pointers and references to static entities; often referred to as non-type parameters)

3.      Other templates

The examples in Volume 1 all fall into the first category and are the most common. The canonical example for simple container-like templates nowadays seems to be a Stack class. Being a container, a Stack object is not concerned with the type of object it stores; the logic of holding objects is independent of the type of objects being held. For this reason you can use a type parameter to represent the contained type:^.

template

class Stack {

  T* data;

  size_t count;

public:

  void push(const T& t);

  // etc.

};

You provide the actual type to be used for a particular Stack instance by means of an argument for the parameter T:

Stack myStack;  // A Stack of ints

The compiler then provides an int-version of Stack by substituting int for T and generating the corresponding code. The name of the class instance generated from the template in this case is Stack^.

Non-type template parameters

It is also possible to provide a non-type template parameter, as long as it represents an integral value that is known at compile time. You can make a fixed-size Stack, for instance, by specifying a non-type parameter to be used as the dimension for the underlying array, as follows^.

template

class Stack {

  T data[N];  // Fixed capacity is N

  size_t count;

public:

  void push(const T& t);

  // etc.

};

You must provide a compile-time constant value for the parameter N when you request an instance of this template, such as

Stack myFixedStack;